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United States Patent |
5,648,187
|
Skotheim
|
July 15, 1997
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Stabilized anode for lithium-polymer batteries
Abstract
The invention relates to thin film solid state electrochemical cells
consisting of a lithium metal anode, a polymer electrolyte and a cathode,
where the lithium anode has been stabilized with a polymer film capable of
transmitting lithium ions.
Inventors:
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Skotheim; Terje Absjorn (Shoreham, NY)
|
Assignee:
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Moltech Corporation (Tucson, AZ)
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Appl. No.:
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618111 |
Filed:
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March 19, 1996 |
Current U.S. Class: |
429/213; 429/303; 429/306 |
Intern'l Class: |
H01M 004/02 |
Field of Search: |
429/213,212,192,194,191,218
|
References Cited
U.S. Patent Documents
4609600 | Sep., 1986 | Heinze et al. | 429/197.
|
4812375 | Mar., 1989 | Foster | 429/101.
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5354631 | Oct., 1994 | Chaloner-Gill et al. | 429/137.
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5387482 | Feb., 1995 | Anani | 424/141.
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5437692 | Aug., 1995 | Dasgupta et al. | 29/623.
|
5460905 | Oct., 1995 | Skotheim | 424/213.
|
5462566 | Oct., 1995 | Skotheim | 29/623.
|
Foreign Patent Documents |
163188 | Sep., 1983 | JP.
| |
Other References
"Electrochemical and Electric Properties of Vacuum-Deposited
Poly(arylene)s: Electrochemical Activity, Diode, and Electroluminescence",
J. Physical Chemistry vol. 96, No. 22, Oct. 29, 1992, 8679-8682.
|
Primary Examiner: Nuzzolillo; M.
Attorney, Agent or Firm: Morrison & Foerster LLP
Parent Case Text
This application is a continuation, of application Ser. No. 197,140, filed
Feb. 16, 1994, now abandoned.
Claims
What is claimed is:
1. A battery cell comprising:
(a) a lithium anode;
(b) a non-aqueous organic electrolyte containing a dissolved lithium salt;
(c) an electrically conducting crosslinked polymer film interposed between
the lithium anode and the electrolyte; said electrically conducting
crosslinked polymer film being capable of transmitting lithium ions
between the lithium anode and the electrolyte; and
(d) a cathode.
2. A battery cell according to claim 1, wherein said electrically
conducting crosslinked polymer film is deposited by vacuum evaporation on
said lithium anode.
3. A battery cell according to claim 1, wherein said electrolyte is a
polymer electrolyte containing a dissolved lithium salt.
4. A battery cell according to claim 3, wherein said polymer electrolyte is
a single-ion conducting polymer electrolyte.
5. A battery cell according to claim 3, wherein the polymer electrolyte is
a gel polymer electrolyte.
6. A battery cell according to claim 3, wherein said electrically
conducting crosslinked polymer film is deposited by vacuum evaporation on
said polymer electrolyte.
7. A battery cell according to claim 1, wherein said electrically
conducting crosslinked polymer film is deposited by plasma-assisted vacuum
evaporation.
8. A battery cell according to claim 1, wherein said electrically
conducting crosslinked polymer film is vacuum deposited using a conjugated
polymer as an evaporation source.
9. A battery cell according to claim 8, wherein said conjugated polymer
source is selected from the group consisting of poly(p-phenylene),
polyacetylene, poly(phenylene vinylene), polyazulene,
poly(perinaphthalene), polyacenes, and poly(naphthalene-2,6-diyl).
10. A battery cell according to claim 1, wherein said lithium salt is
selected from LiCF.sub.3 SO.sub.3, LiClO.sub.4, LiAsF.sub.6, LiPF.sub.6,
LiN(SO.sub.2 CF.sub.3).sub.2, LiC(SO.sub.2 CF.sub.3).sub.3 and lithium
salts of fluorosulfonated phenols and pyrroles.
11. A battery cell according to claim 1, wherein said cathode comprises a
cathode active material selected from inorganic insertion oxides and
sulfides, organo-sulfur compounds and conjugated polymers.
12. A battery comprising a battery cell according to claim 1 which further
comprises a current collector for said lithium anode, a separate current
collector for said cathode and suitable encapsulation to prevent the
penetration of air and moisture.
13. A battery cell according to claim 1, wherein said electrically
conducting crosslinked polymer film is less than 10 micrometers thick.
14. A battery cell according to claim 13, wherein said electrically
conducting crosslinked polymer film is 0.1 to 5 micrometers thick.
15. A battery cell according to claim 1, wherein said electrically
conducting crosslinked polymer film is deposited by plasma-assisted vacuum
evaporation using conjugated oligomers as an evaporation source.
16. A battery cell according to claim 15, wherein said conjugated oligomers
are phenyl oligomers.
17. A battery cell comprising:
(a) a lithium anode;
(b) a non-aqueous solid polymer electrolyte containing a dissolved lithium
salt;
(c) a lithium ion conducting polymer film interposed between the lithium
anode and the electrolyte; said polymer film being doped electrically
conductive and capable of transmitting lithium ions between the lithium
anode and the electrolyte by incorporation of lithium ions, wherein said
lithium ion-doped polymer film is capable of stabilizing the lithium anode
against the formation of dendrites and has the capability to dissolve
dendrites and further is capable of stabilizing the lithium anode against
reaction with said polymer electrolyte to form a more resistive
interfacial layer; and
(d) a cathode.
18. A battery cell according to claim 17, wherein said lithium ion
conducting polymer film is deposited by vacuum evaporation on said lithium
anode.
19. A battery cell according to claim 17, wherein said polymer electrolyte
is a single-ion conducting polymer electrolyte.
20. A battery cell according to claim 17, wherein said polymer electrolyte
is a gel polymer electrolyte.
21. A battery cell according to claim 17, wherein said lithium ion
conducting polymer film is deposited by vacuum evaporation on said polymer
electrolyte.
22. A battery cell according to claim 17, wherein said lithium ion
conducting polymer film is deposited by plasma-assisted vacuum
evaporation.
23. A battery cell according to claim 17, wherein said lithium ion
conducting polymer film is vacuum deposited using a conjugated polymer as
an evaporation source.
24. A battery cell according to claim 23, wherein said conjugated polymer
source is selected from the group consisting of poly(p-phenylene),
polyacetylene, poly(phenylene vinylene), polyazulene,
poly(perinaphthlene), polyacenes, and poly(naphthalene-2,6-diyl).
25. A battery cell according to claim 17, wherein said lithium salt is
selected from LiCF.sub.3 SO.sub.3, LiClO.sub.4, LiAsF.sub.6, LiPF.sub.6,
LiN(SO.sub.2 CF.sub.3).sub.2, LiC(SO.sub.2 CF.sub.3).sub.3, and lithium
salts of fluorosulfonated phenols and pyrroles.
26. A battery cell according to claim 17, wherein said cathode comprises a
cathode active material selected from inorganic insertion oxides and
sulfides, organo-sulfur compounds and conjugated polymers.
27. A battery comprising a battery cell according to claim 17 which further
comprises a current collector for said lithium anode, a separate current
collector for said cathode and suitable encapsulation to prevent the
penetration of air and moisture.
28. A battery cell according to claim 17, wherein said lithium ion
conducting polymer film is less than 10 micrometers thick.
29. A battery cell according to claim 28, wherein said film is 0.1 to 5.0
micrometers thick.
30. A battery cell according to claim 17, wherein said lithium ion
conducting polymer film is vacuum deposited using a conjugated polymer as
an evaporation source and said lithium ion conducting polymer film
comprises a conducting polymer with a different chemical structure than
said conjugated polymer source.
31. A battery cell according to claim 30, wherein said conjugated polymer
source is selected from the group consisting of poly(p-phenylene),
polyacetylene, poly(phenylene vinylene), polyazulene,
poly(perinaphthalene), polyacenes, and poly(naphthalene-2,6-diyl).
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to thin film solid state electrochemical cells. More
particularly, this invention relates to novel stabilized negative
electrodes for electrochemical cells, consisting of a lithium metal coated
with a thin film of an electroactive polymer capable of transmitting
alkali metal ions interposed between the lithium anode and the polymer
electrolyte.
2. Prior Art
Rechargeable lithium-polymer batteries are promising advanced power sources
for a variety of applications such as to portable electronic devices and
electric vehicles. Although a variety of polymer electrolytes have been
investigated as the ionic conducting medium, the reactivity of lithium
metal has posed a formidable challenge to develop polymer electrolytes
which have the requisite chemical and electrochemical stability for long
cycle life [F. M. Gray, Solid Polymer Electrolytes (VCH Publishers, Inc.,
New York 1991)].
A key problem with lithium metal anodes is the formation of dendrites upon
repeated plating of lithium metal during charging of the battery. This has
led to a detailed investigation of lithium alloys, such as
lithium-aluminum alloys, and lithium-carbon composites as alternatives to
lithium metal anodes. U.S. Pat. No. 4,002,492 discloses electrochemical
cells having anodes consisting of lithium-aluminum alloys where the
content of lithium is from 63% to 92%. Other disclosures of
lithium-aluminum anodes are found in Rao, et al., J. Electrochem. Soc.
124, 1490 (1977), and Besenhard, J. Electrochem. Soc., 94, 77 (1978). The
use of lithium-carbon composite anodes is disclosed in Ozawa et al., in
Proc. Tenth International Seminar on Primary and Secondary Battery
Technology, Deerfield Beach, Fla., March 1993.
The central problem with composite lithium anodes is an increase in weight
and volume due to the addition of non-electroactive materials. In the case
of lithium-aluminum alloys, there is also a loss in potential of about 0.4
V. The loss in cell voltage coupled with increased weight implies a
significant loss in specific energy of the cell. Batteries using
lithium-aluminum alloys as anodes have exhibited relatively low
capacities, low rate capabilities and poor cycle life.
Lithium-carbon composites based on intercalation in graphitic carbon
generally have a voltage drop of 0.3 V-0.5 V vs. lithium and typically
involve 8-10 carbon atoms for each lithium atom, the theoretical maximum
being 6 carbon atoms for each lithium atom. This entails a significant
penalty in increased weight and volume, and, consequently, decreased
capacity. Cells using lithium-carbon composite anodes have, however,
demonstrated long cycle life, with more than 1,000 cycles recorded.
Shacklette, et al., disclose the use of a conjugated polymer-lithium
composite anode in U.S. Pat. No. 4,695,521, which incorporates an n-doped
conjugated polymer as a substrate for electroplating a lithium metal,
resulting in finely divided lithium metal distributed throughout a
conducting polymer matrix. Cells incorporating conjugated polymer-lithium
composite anodes have long cycle life, but reduced capacity. The n-doped
conjugated polymers have low capacity that limits the capacity of the
anode material.
Toyoguchi, et al., disclose the use of a prefabricated film of a conjugated
polymer to coat the lithium surface of an anode in a cell using a liquid
organic electrolyte in kokai 58-163188 (1983). Cells with lithium anodes
coated with a conjugated polymer showed enhanced cycling ability compared
with equivalent cells using bare lithium anodes. Prefabricated conjugated
polymer films are highly porous and at least 10 micrometers thick in order
to have sufficient mechanical strength to be free-standing. Porous films
are not suitable if the electrolyte is polymeric since a polymer
electrolyte cannot penetrate the pores of the film, resulting in inferior
contact between the conjugated polymer and the electrolyte. With liquid
organic electrolyte, a porous film does not provide complete surface
coverage, and therefore not as complete protection as a dense polymer
film. The relatively thick prefabricated conjugated polymer films also add
significant weight and volume to the cell, limiting the capacity of the
cell.
There is a clearly defined need, therefore, for novel concepts in
interfacial engineering of the lithium-electrolyte interface that allows
the fabrication of rechargeable lithium cells having long cycle life and
incorporating polymer electrolytes.
SUMMARY OF THE PRESENT INVENTION
The present invention obviates one or more of the disadvantages of
electrochemical cells using anodes made from lithium alloys,
lithium-carbon composites and lithium-conducting polymer composites, by
providing a lithium metal anode that has been stabilized against dendrite
formation by the use of a vacuum evaporated thin film of a lithium-ion
conducting polymer interposed between the lithium metal and the
electrolyte. The present invention also provides a rechargeable, high
energy density electrochemical battery cell that contains:
(a) a lithium anode;
(b) a thin film of a lithium metal-ion conducting polymer which is doped
n-type by the incorporation of lithium ions and which is deposited on the
lithium anode surface by vacuum evaporation;
(c) a non-aqueous liquid or polymeric electrolyte containing a lithium salt
dissolved therein; and
(d) a cathode containing a cathode active material.
As a result of the present invention, rechargeable lithium-cells are
provided having a higher energy density and longer cycle life than has
previously been achieved.
For a better understanding of the present invention, reference is made to
the following description and the accompanying drawings. The scope of the
invention will be pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic structure of a cell incorporating a lithium-ion
conducting film interposed between the lithium metal anode and the polymer
electrolyte.
FIG. 2 shows infrared (IR) spectra of poly(p-phenylene) powder in KBr (A)
and a PPP film evaporated onto a silicon wafer (B).
FIGS. 3A and 3B show ac impedance spectra as a function of time for
symmetrical cells Li/SPE/Li (3A) and Li/PPP/SPE/PPP/Li (3B), where SPE
designates a Solid Polymer Electrolyte, and PPP designates an evaporated
interfacial film where poly(p-phenylene) was the starting material for the
evaporation.
FIGS. 4A and 4B show current-voltage effects on Li/SPE/Li (.largecircle.)
and Li/PPP/SPE/PPP/Li (.circle-solid.) cells as a function of time, for 10
mV and 20 mV (4A) and 45 mV (4B).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The novel cell of this invention contains as an essential element a vacuum
evaporated polymer film interposed between a lithium metal anode and an
electrolyte, where the polymer film has the capability to transmit lithium
ions and to reduce the formation of dendrites on the lithium surface. The
vacuum evaporated film is dense and provides complete surface coverage.
Certain conjugated polymers, such as polyacetylene and poly(p-phenylene),
can be doped with lithium ions to be electrically conductive. In the form
of thin films, the polymers have the ability to transmit lithium ions by
diffusion as demonstrated by their electrochemical reversibility for
lithium ion insertion and de-insertion. Thin films of the conjugated
polymers can be doped by lithium ions by contacting them with lithium
metal. The lithium metal dissolves lithium ions which diffuse into the
polymer structure up to a certain maximum concentration.
In the present invention, a thin film of a conjugated polymeric structure
is placed at the interface between the lithium metal anode and the polymer
electrolyte. With a bare lithium anode, the charge/discharge reaction at
the anode of the cell is
Li.revreaction.Li.sup.+ +e.sup.-
With the lithium electrode coated with a lithium ion transmitting polymer
layer, the charge/discharge reaction becomes
Li.revreaction.Li.sup.+ (polymer).sup.- .revreaction.Li.sup.+ +e.sup.-
Charging and discharging reactions at a bare lithium surface in an organic
electrolyte, leads to irreversible changes of the lithium surface. Upon
stripping of lithium metal into lithium ions in the electrolyte, the
dissolution occurs non-uniformly across the surface, leading to a
roughening of the surface. Upon re-deposition of lithium ions onto the
lithium metal surface, the plating does not occur uniformly but
preferentially at the protruding points on the surface, leading ultimately
to the formation of dendrites and a shorting of the cell. Some dendritic
material will be encapsulated and no longer accessible for cycling,
requiring an excess of lithium. The high surface area of the lithium that
results from the dendritic growth and subsequent encapsulation poses
severe safety hazards.
When the lithium surface is coated with an evaporated lithium ion
conducting polymer film, the film is doped by dissolution of lithium into
lithium ions which diffuse into the film until a maximum concentration is
reached. This renders the polymer film electrically conducting and capable
of transmitting lithium ions between the lithium metal anode and the
electrolyte. During the discharge process, lithium ions will enter the
electrolyte from the polymer film and lithium metal ions will dissolve
into the polymer film at the lithium-polymer interface. The lithium-doped
polymer film provides a constant potential across the lithium metal
surface due to its high electrical conductivity, thereby providing
thermodynamically more favorable stripping conditions without pitting of
the electrode surface. In addition, the polymer film has the ability to
dissolve micro-dendrites as they are formed.
Similarly, during the plating of lithium from the electrolyte, the plating
takes place solely at the lithium-conducting polymer interface where a
uniform potential is maintained. Therefore, no preferential deposition
occurs at protruding areas of the lithium surface.
The ability to prevent the formation of dendrites that can electrically
short the cell, allows the fabrication of cells with thinner polymer
electrolyte films. This provides cells with higher capacity for energy
storage by weight and volume.
FIG. 1 shows schematically the structure of a cell incorporating an
electrically conducting film capable of transmitting lithium ions
interposed between the lithium metal anode and the electrolyte. The
thickness of the electroconducting film is from 0.01 .mu.m to 10 .mu.m,
with the preferred thickness being from 0.1 .mu.m to 5 .mu.m.
Evaporation in a vacuum is the preferred method of deposition of the
electroconductive film. Vacuum deposition provides dense films and
complete surface coverage. If the vacuum deposition uses low molecular
monomeric or oligomeric materials as evaporation sources, the evaporation
may be performed through a plasma which polymerizes the monomers or
oligomers to provide an insoluble film on the lithium surface.
The electroconductive film may be evaporated onto the lithium surface
followed by lamination of the polymer coated anode with the polymer
electrolyte, or deposition of the polymer electrolyte onto the
electroconductive film by other means, such as extrusion. Alternatively,
the electroconductive film may be evaporated onto the polymer electrolyte,
followed by subsequent lamination with a lithium foil electrode, or
thermal evaporation of the lithium electrode onto the electroconductive
film in a vacuum.
Useful starting polymers for the formation of the electroconductive polymer
film may be any conjugated structure which is capable of being doped
electrically conductive by lithium ions, such as poly(p-phenylene),
polyacetylene, poly(phenylene vinylene), polyazulene,
poly(perinaphthalene) polyacenes and poly(naphthalene-2,6-diyl). This list
of polymers is illustrative and not intended to be exhaustive. Amongst
these illustrative conjugated polymers, poly(p-phenylene) is preferred.
When the polymer film is deposited by vacuum evaporation the resulting
polymer may have a structure which is different from the starting
material. FIG. 2 shows the IR spectrum of a poly(p-phenylene) film mixed
in powder form in a KBr pellet (A) compared with the IR spectrum of a 0.1
.mu.m film deposited on a silicon wafer by thermal vacuum evaporation
using poly(p-phenylene) as the starting material. The starting polymer is
decomposed thermally during the evaporation process. The polymer
decomposition leads to formation of highly reactive fragments of lower
molecular weight. Polymerization occurs by recombination of these reactive
fragments on the substrate surface. The deposited film is not
poly(p-phenylene), as can be clearly seen from the IR spectra, but a
highly crosslinked and branched electroconductive polymer film which can
be doped with lithium ions. Vacuum deposition of electroconductive films
using conjugated polymers as starting materials have been disclosed by
Yamamoto et al., in J. Physical Chemistry, vol. 96, p. 8677 (1992).
Similarly, films may be produced by using oligomers of varying molecular
weight as starting material. If crosslinked, insoluble films are desired,
the oligomer vacuum evaporation may be performed through a plasma which
generates reactive groups that lead to polymerization on the substrate
surface.
The electrolyte may be a thin film of a solid polymer electrolyte, such as
an amorphous polyether, a branched polysiloxane with ethylene oxide side
chains or a branched polyphosphazene with ethylene oxide side chains, into
which is dissolved a lithium salt. The conductivity of the polymer
electrolyte may be enhanced by the addition of plasticizing compounds of
low molecular weight, such as propylene carbonate, ethylene carbonate,
N-methyl acetamide. sulfonate, sulfolane, 1,2-dimethoxyethane,
poly(ethylene glycol), 1,3-dioxolane and glymes. Plasticized polymer
electrolytes are also known as gel polymer electrolytes. The polymer
electrolyte may be an exclusive cation conducting polymer electrolyte, a
so-called single-ion conductor, wherein the anionic charges are covalently
attached to the polymer backbone. The conductivity of the single-ion
conducting polymer electrolytes may be enhanced by the addition of
plasticizing compounds. Useful lithium salts are LiCF.sub.3 SO.sub.3,
LiClO.sub.4, LiAsF.sub.6, LiPF.sub.6, LiN(SO.sub.2 CF.sub.3).sub.2,
LiC(SO.sub.2 CF.sub.3).sub.3 and lithium salts of fluorosulfonated phenols
and pyrroles. The preferred thickness of the polymer electrolyte is from 1
.mu.m to 50 .mu.m, most preferably from 1 .mu.m to 25 .mu.m.
Suitable cathode active materials can be selected from the group of
inorganic insertion oxides and sulfides, organo-sulfur compounds and
conjugated polymers. Useful inorganic insertion oxides include CoO.sub.2,
NiO.sub.2, MnO.sub.2, Mn.sub.2 O.sub.4, V.sub.6 O.sub.13 and V.sub.2
O.sub.5. Useful inorganic sulfides include TiS.sub.2 and MoS.sub.2. Useful
organo-sulfur materials include polymerization/depolymerization compounds
as disclosed in U.S. Pat. No.4,833,048 and polymers such as poly(carbon
disulfide). Suitable conjugated polymers include polyacetylene,
poly(phenylene vinylene) and polyaniline. Typically, the cathode is a
composite material consisting of cathode active material (40-70%), polymer
electrolyte for ionic conductivity (20-50%) and carbon black for
electronic conductivity (5-20%). The composite cathode may also contain a
small fraction (1-5%) of a binder, for example teflon, for mechanical
stability.
Details of the preferred embodiments have been set forth herein in the form
of examples which are described below.
EXAMPLES
Example 1
Interface Stability Studied by ac Impedance Spectroscopy
A 0.1 micron film of PPP was deposited by thermal vacuum evaporation onto
two lithium foils using poly(p-phenylene) as the starting material. The
vacuum chamber was placed inside an argon atmosphere glove box. The
residual pressure in the chamber was 0.01-0.02 torr and the evaporation
temperature 300 deg.-350 deg. C. Symmetrical cells of the construction
Li/SPE/Li and Li/PPP/SPE/PPP/Li were constructed, where SPE designates
solid polymer electrolyte and PPP is the resulting film deposited by
thermal evaporation of poly(p-phenylene). The SPE consisted of a 25 micron
thick Celgard 2500 membrane soaked in a liquid branched polysiloxane with
LiCF.sub.3 SO.sub.2 salt, where the Li/O ratio was 1/24. FIG. 3 shows the
ac impedance spectra of the two symmetrical cells as a function of time.
The time evolution of the ac impedance spectrum of the Li/SPE/Li cell (3A)
shows a lack of stabilization even after long times. This implies a
continuing chemical reaction between the lithium electrode and the polymer
electrolyte resulting in the build-up of a highly resistive interfacial
layer. With the PPP interfacial film (3B) the ac impedance spectra show an
initial increase in the interfacial resistance at short times followed by
stabilization.
Example 2
Interfacial Stability Studied by Current-Voltage Characteristics
Symmetrical cells were constructed as in Example 1. The stability of the
lithium-SPE and the lithium-PPP-SPE interfaces was studied under
conditions of direct current passing through the interface. Constant
potentials were maintained across the cells and the currents were
monitored. Voltages of 10 mV, 20 mV and 45 mV were used. The results are
shown in FIGS. 4A and 4B. The current passing through the Li/SPE/Li cell
continues to drop for the duration of the experiment, whereas the current
passing through the Li/PPP/SPE/PPP/Li cell levels off after an initial
decay. A continuous decrease in the current with time with constant
potential implies that the interfacial resistance increases.
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